What Happens When a Super Storm Strikes New York?

Last year New York City escaped the worst of Hurricane Irene. Now, the "frankenstorm" combination of Hurricane Sandy and other storm systems are bearing down on the Northeast. Back in 2011, using U.S. Army Corps of Engineers' calculations, PM examined how much damage a direct hit by a hurricane could cause to New York City. Here's what super storms are made of—and how the whole country can prepare for the worst.

The hurricane churning east of New Jersey seems destined for the mid-Atlantic. Then a cold front descending out of Canada nudges the Category 2 storm northwest instead—setting it on a worst-case course for New York City.

New York Harbor has often sheltered the city, dissipating energy from violent gales that start at sea. But now it plays an opposite role: It turns an otherwise moderate hurricane into a disaster. As the eye of the storm passes over Staten Island, the 100-mph counterclockwise winds shove 500 million tons of seawater directly into the harbor. The narrowing shorelines and shallowing sea bottom cause the mass of water to build. By the time the storm surge washes over the shores of Brooklyn, Queens and Manhattan, it towers 11 to 15 feet high.

Water flows through New York's financial district and reaches 2 miles into southern Brooklyn and Queens, flooding 2900 miles of roads. Impromptu rivers gush into subway stations and pour through hundreds of sidewalk gratings.

In Manhattan, the lower levels of Penn Station and Grand Central fill with water. The subway floods within 40 minutes—paralyzing the city's chief form of public transportation. Three of the four automobile tunnels linking Manhattan to the outer boroughs and New Jersey also flood, submerging hundreds of cars stranded in traffic jams during evacuation. A million people lose electricity and phone service as floods shut down 10 power plants and the emergency generators powering cellphone towers.

While this scenario may sound like yet another apocalypse-in-New York summer blockbuster, it was produced using calculations from the U.S. Army Corps of Engineers—and it's been given serious attention from government planners. That 1995 Army Corps report and a 2006 analysis by the Department of Homeland Security predict that a Category 4 hurricane scoring a direct hit on New York City would inflict $500 billion worth of damage—quadruple that wrought by Category 5 Hurricane Katrina in 2005.

A third study, released this September by New York state, predicts that an even milder, Category 1 hurricane or winter nor'easter could inundate the city's subway and cause $58 billion in losses. Experts don't consider such disastrous flooding a mere possibility; they believe it's a certainty—a one-in-100-year event. Sea level rise will upgrade it to a one-in-35-year event by 2080.

"We've been very, very lucky because we haven't had that [direct hit]," says Cynthia Rosenzweig, a climate-impact scientist at the NASA Goddard Institute for Space Studies in New York who has helped guide the city's storm- and climate-­planning effort. "But the potential vulnerability for that is very high."

New York City

According to the Metro New York Hurricane Transportation Study, an analysis by the U.S. Army Corps of Engineers, a Category 4 hurricane could inflict $500 billion worth of damage. SLOSH computer models from the city's Office of Emergency Management show that a direct hit by even a Category 2 storm would completely inundate Rockaway Peninsula; a Category 3 storm would put JFK airport under 19 feet of water.

Every region of the U.S. is subject to catastrophic storms of one type or another. While the severe floods and tornadoes that devastated large swaths of the country this spring surprised many people, there's no reason they should have. Annual losses from natural hazards have increased severalfold over time—costing the nation $573 billion in crops and property since 1960. Americans are turning even routine storms into full-blown disasters by settling where they strike. Then, when vulnerable infrastructure is swept away, people have exhibited a steadfast commitment to rebuilding it.

"There are more people living in what we might consider to be high-hazard areas," says Susan Cutter, a disaster scientist at the University of South Carolina in Columbia. These include coastal areas, floodplains and places especially prone to tornadoes and landslides. By 2040, 70 percent of the U.S. population—which should then number 400 million—is expected to concentrate in 11 megaregions, seven of which occupy coastal counties.

If New York—part of the Northeast megaregion—suffers a direct hit, workers will spend weeks pumping a billion gallons of brackish water out of its subway and train tunnels. The salt will corrode power lines, transformers and thousands of brakes and switches that control the trains. Some subsystems could take a year or more to restore.

To avoid such a scenario, New York state recommends the city invest well over $100 million a year in storm protections. City planners are already experimenting with dozens of low-tech fixes, says Adam Freed, deputy director of the Mayor's Office of Long-Term Planning and Sustainability. These include raising subway vents above sidewalks, installing several-inch-high barriers around subway entrances and using porous pavement. They've also considered building lips around rooftops to slow the percolation of water into streets and sewers, because every inch of rain that falls on New York translates to a billion gallons of storm water that must be managed.

Some observers, such as Malcolm Bowman, an oceanographer at the State University of New York at Stony Brook, have even suggested that four massive barriers be built across the waterways surrounding the city. The arms would swing shut during severe storms—much like those of the Maeslantkering, a barrier that protects the Port of Rotterdam from surges in the North Sea.

Canton, Mo., a town of 2500 on the upper Mississippi River, has been at the center of an increasingly high-stakes environmental wager for years now. In the summer of 1993, a high-pressure system stalled over the southeastern U.S., forcing the jet stream, laden with moist air from the Gulf of Mexico, to the north, where it collided with cold air from Canada. As a result, rainstorms drenched the upper Midwest. Many towns received two to six times the normal amount of rainfall for June and July.

The Mississippi River crested at 13.8 feet above official flood levels in Canton, overtopping several local levees. That year, more than 1000 levees ruptured or overflowed along the Mississippi and Missouri rivers. Seventy towns, including Canton, flooded. The water stayed high for six months.

According to government statistics, the flood that Canton experienced in 1993 was a freak, one-in-500-year event—not something that would happen again soon. That estimate came from analyzing the 140-year historical record—calculating the frequency of floods of various magnitudes and extrapolating the curve out to events at a scale never seen before. If only it were that simple. Canton suffered another 500-year flood in 2008, a 70-year flood in 2001, and 10-year floods in 1996, 1998 and again in early 2011. Plenty of towns across the region have suffered similar events.

"We're witnessing higher and higher floods over time," says Robert Criss, a hydrogeologist at Washington University in St. Louis. "We are seeing higher and far more frequent floods than government estimators say we should."

The data are too noisy to chalk that trend up to increased rainfall. Instead, official statistics may underestimate the severity of floods in this region because records are too short to reveal the full variability of the climate. "We have no idea what Mother Nature is capable of dishing out," Criss says.

People tend to view earthquakes and hurricanes as the most damaging natural disasters—but a steady rain could do far worse. In the winter of 1861 to 1862, California experienced a series of rainstorms lasting 45 days. The Central Valley became a shallow lake that lingered for months. Newspapers described people traveling the streets of Sacramento in boats.

A team of 40 scientists recently modeled the effects of such a roughly 500-year storm if it were to strike California today. "There's no way that the magnitude of the storm and the subsequent flooding could be contained by the existing flood structures," says Justin Ferris, a hydrologist at the U.S. Geological Survey's California Water Science Center in Sacramento. "Such a flood would be devastating."

Ferris and others estimate that a 300- by 20-mile swath of the Central Valley would flood. Waterlogged soil would trigger hundreds of landslides. While the USGS considers a magnitude 7.8 earthquake along the San Andreas fault an equally likely event, the California storm would cost nearly three times as much—$300 billion in direct damages.

Plenty could be done to soften the impact of massive downpours, but it will mean undoing 150 years of misguided policy. Engineers have progressively walled in the upper Mississippi and lower Missouri rivers as they straightened them for ship navigation—in some places decreasing the rivers' width by two-thirds since 1875. This reduced their ability to expand during floods. Compared with a century ago, an equivalent amount of water flowing down the upper Mississippi River now causes the water to rise 10 to 12 feet higher.

In Chesterfield Valley, Mo., malls and homes worth several billion dollars have been constructed in the past decade on land that was underwater in 1993—requiring the government to build up levees. Breaking that cycle, Criss says, will mean putting an end to misleading flood-risk statistics and the artificially cheap federal flood insurance that goes hand in hand with them. "It enables people to obtain financing for very economically damaging projects," he says. "It puts the taxpayer on the short end of the stick."

You might say the Army Corps of Engineers took a small step in the right direction on May 3, 2011. That's when it dynamited a 2-mile section of levee on the Mississippi River to divert water onto farmland and save the town of Cairo, Ill., from flooding. But the ensuing wall of water inflicted long-term damage—scouring away topsoil, gouging gullies 8 feet deep and dumping sand.

A better approach is to plan for flooding by building lower levees that are designed to overflow, allowing the farmland to flood more often—and more gently. "We need a system that uses farmland for floodwater storage," Criss says. "It will help the environment, and the farmers can be compensated for that, just like we compensate them for letting land lie fallow." Such a system would pay for itself by reducing rebuilding costs; flood damages currently total $1 billion to $10 billion per year in the U.S.

Flood and hurricane risk can at least be predicted: It is heavily influenced by topography, and the storms and floodwaters can be tracked for days in advance. But severe tornadoes, like the ones that tore across the central and eastern U.S. in 2011, pose a very different challenge.

The tornado that ripped through Joplin, Mo., on May 22, a 5 on the enhanced Fujita (EF) scale, existed for just 38 minutes.

During that time it plowed a path three-­quarters of a mile wide through town, destroying nearly 7000 homes and tossing trucks 200 yards. Many people, such as Pizza Hut manager Christopher Lucas, reacted just in time: He crowded 19 people into his walk-in freezer. They survived, although Lucas—sucked out of the freezer as he held the door shut with a bungee cord—was among the 150 who died.

The tornado seemingly could have struck anywhere. The moist air that flowed from the Gulf of Mexico that day created a whole herd of potentially tornado-­spawning thunderstorms, from Oklahoma to Minnesota. The Joplin tornado affected just a few square miles of that vast area—yet it did so with overwhelming fury. Its winds, over 200 mph, hammered buildings with four times the pressure that a Category 2 hurricane with 100-mph winds would have exerted.

It's possible to build a house to withstand 100-mph winds, providing partial protection against some weaker tornadoes. But 200-mph winds? "It's just not practical to design the entire building to withstand those kinds of pressures," says Ernst Kiesling, an engineer at Texas Tech University's Wind Science and Engineering Research Center. "It would be too expensive." Even if houses can't be protected from EF5 tornadoes, Kiesling has spent decades looking for ways to save the lives of the people inside them.

After a tornado killed 26 people and destroyed hundreds of homes in Lubbock, Texas, in 1970, Kiesling and his colleagues noticed a curious thing: Even in buildings that were blown apart, an interior bathroom or closet was sometimes left intact. It gave Kiesling an idea: Convert a small, windowless room in the house into a tornado shelter that could survive 250-mph winds. Many of the fatalities in tornadoes occur when people are struck by projectile debris—the Joplin tornado, for example, drove all four legs of a chair through the walls of one house. So Kiesling's team tested their shelters with a gun that fired 2 x 4s at 100 mph (the speed at which they would be propelled by 250-mph winds). They settled on steel-reinforced plywood to make the structures puncture-proof. Such shelters can now be installed for $3000 to $10,000.

Researchers are working on other technical solutions to tornado protection—for example, radar that provides earlier warnings. But societal trends continually work against even the best of efforts. "For any intensity of tornado, you're more likely to be killed if you're in a mobile home than in a permanent home—15 to 20 times more likely," says Harold Brooks of the National Severe Storms Laboratory in Norman, Okla. Unfortunately, the proportion of Americans living in mobile homes has tripled since 1970 (and it is highest—about 15 percent—in the tornado-prone Southeast).

In this sense, the problems posed by tornadoes do bear a resemblance to those of hurricanes, floods and other severe storms. Some steps needed to minimize losses, such as stricter building codes and sturdier infrastructure, are well-known. They require greater investment. Other solutions, just as important, involve choosing what not to protect. Instead of applying brute force, they'll mean removing the perverse incentives that encourage people to build in high-risk areas. Because for those who find themselves in harm's way, even modest storms can be super.